1. Field of the Invention
Embodiments of the invention relate to a synthetic aperture radar (SAR) system with an antenna that includes one or several partial antennas. Each partial antenna includes a plurality of phase centers and transmit/receive modules assigned thereto and a signal processor for the coherent processing of signals of the phase centers or radiators.
2. Discussion of Background Information
A SAR system of this type is used, for example, with space-based SAR instruments. FIG. 1 diagrammatically represents a planar phased array antenna ANT. Antenna ANT may include, by way of non-limiting example, six panels, i.e., Pan1-Pan6, as partial antennas with respectively 56 radiators Rad as phase centers and a corresponding number of transmit/receive modules TRM. Electronic components of antenna ANT, and in particular components for digital beam forming (DBF), are not illustrated. Each of panels Pan1-Pan6 in the exemplary embodiment includes two radiators Rad in the horizontal direction (typically in the direction of flight) and 28 horizontal radiator pairs in the vertical direction (elevation). In the embodiment of FIG. 1, the horizontal direction runs from left to right, and the vertical direction runs from top to bottom. Radiators Rad have their feed points for transmit signals or receive signals respectively in the center. The already mentioned transmit/receive module TRM is arranged behind each of the feed points.
Each transmit/receive unit TRM has on an instrument side an input for a high-frequency transmit signal (HF transmit signal) and an output for a high-frequency receive signal (HF receive signal). The HF transmit signal is referred to as a radar transmit pulse, and the HF receive signal is referred to as a radar echo. In the present specification, each transmit/receive module TRM represents an individual channel. In principle, it is understood that several transmit/receive modules can be combined to form a channel. The transmit/receive modules TRM are controlled, for example, via electrical line buses, such as antenna timing bus (ATB) and an antenna control bus (ACB). In phased array antennas for SAR systems, a HF network for transmit signal distribution and at least one further network for the receive signal combination is typically provided on the antenna. FIG. 2 diagrammatically illustrates the HF network of a typical antenna for only eight radiators Rad, but without digital beam formation. For better clarity, the eight radiators Rad are shown with vertical spacing from one another. The transmit/receive modules TRM are coupled to a central electronic system COMP. If a signal is to be transmitted by the radiator Rad of the antenna ANT, this is fed via an output RF TX as a transmit signal to the transmit/receive modules TRM. An analogously combined receive signal of the transmit/receive modules TRM, which is formed from the respective signals of the transmit/receive modules TRM of the radiators Rad, is digitalized, further processed and finally recorded in the central electronic system COMP arranged apart from the antenna. The combined receive signal is fed to the central electronic system COMP at an input RF RX.
It is known to use digital beam forming to increase the power of radar antennas. With SAR systems this is used, for example, in high resolution wide swath (HRWS) instruments. One possible embodiment is described in EP 1 241 487 A1, the disclosure of which is expressly incorporated by reference herein in its entirety. In digital beam forming, the receive signals of all channels (in the exemplary embodiment, all transmit/receive modules) are processed before combination. The processing or handling of the receive signals is usually carried out digitally. A non-limiting exemplary HRWS architecture is shown diagrammatically in FIG. 3, however, the radiators are not shown for reasons of clarity and ease of explanation. The analog receive signals originating from the transmit/receive modules TRM are respectively digitalized and processed in a digital beam forming module DBFM and conveyed as a data stream data RX to the central electronic system. In this way, the data stream is realized as a chain, since the signal combination after the digitalization is a summation. In each beam forming module DBFM, the individual signal and the signal of the predecessor in the chain are added before being forwarded.
The functions of the beam forming module DBFM utilize a signal filtering, a down-mixing of the signal to the baseband, an anti-alias filtering, a digitalization, an application of the beam forming algorithms for the current channel on the digital signal, a coherent, i.e., synchronous summation of the local signal with that of the preceding signal and the forwarding of the sum signal to the next channel, i.e., forwarding to the following beam forming module in the chain. The components FIL, MIX, AAL, ADW and Proc for performing the process are illustrated in FIG. 4 albeit without detailed consideration of an amplifier arrangement. The architecture of a single channel is thereby shown without variable damping elements and phase control elements. The signal data out resulting at the end is the signal in the baseband which a receive antenna would generate with the algorithmically adjusted receive beam.
The described procedure described in EP 1 241 487 A1 contains the early digitalization of the signals already on or at the output of the antenna. As a result, the receive-side architecture of the antenna changes fundamentally compared to a classic antenna without beam formation. The analog receive network used there is replaced by so-called high-speed serial links (HSSL), that is, quick serial data lines. This results from the fact that large data volumes at a high data rate have to be transmitted from the antenna to the central electronic system or directly to the mass storage device. Depending on the bandwidth of the radar, data rates per panel of several 10 Gbps (10×109 bits per second) can occur.
This leads to a high implementation expenditure in particular with spaceborne-capable components with many parallel lines and considerable data processing expenditure. For HRWS instruments with high resolution, very powerful and expensive processing units, generally field-programmable gate array (FPGAs), are used for the data processing, however, the airworthiness of these units have not been unreservedly acknowledged. Furthermore, the high data rates of the receive chain lead to complex cable runs and high power consumption. A further problem of the architecture according to FIG. 3 is the fault dependency of the components on one another. A single error in the HSSL data chain brings the function of the entire panel to a stop. The architecture in the form shown is therefore not airworthy and requires a redundancy concept, which further increases the complexity in the receive chain.
A SAR system with digital beam forming modules is known from the publication FISCHER, C. et al.: Development of a High-Resolution Wide Swath SAR Demonstrator in: 8th European Conference on Synthetic Aperture Radar (EUSAR), 2010, p. 1166-1169, the disclosure of which is expressly incorporated by reference herein in its entirety. A beam sum formation for obtaining an output signal within the scope of the digital beam forming method is carried out in the digital range.